I have used many, but not had to buy them myself, the insulation tester has to use 500 volt there are some with a 250 volt option, all seem to work nearly the same way, continuity tester the same seem to remember 250 mA has to be used and that that much in them.
The loop impedance test however does vary, there are different methods to allow testing without tripping the RCD and some used just 9 mA other can use more around 10 amp, also some are 230 volt only and some 230 or 400 volt and some old ones may still work on 240 volt, also when switching between PSCC and loop impedance some auto change the terminals on the ISO plug so test line - earth with loop and line neutral with PSCC other you need to make up a box of testing leads to do the full test.
The RCD tester also varies, most will test the standard 30 mA, 100 mA, 300 mA and 500 mA RCD's with half, full and 5 times test figures, and of course measure time, others will do a ramp test so you can see exactly at what point it tripped.
As to calibration not a clue what it costs, or if a all in one test set is more or less to get tested to a three meter test set, a lot of all in one test sets also will test an earth electrode with the two probes.
Having sent meters away for calibration and latter found they were completely out due to being set for an old redundant set of regulations rather than anything wrong I am not too happy at using authorised testing houses, I prefer to test my self, however it's not down to what I want it is down to what those passing the paperwork will accept.
So if I make out an insulation certificate and submit it to the LABC they could re-test some random items, they could ask to see calibration certificates, and also could ask to see your qualifications to show you have the skill. I can only say what has happened to me and my son, we both have the C&G2391 and in Liverpool and Cheshire the LABC allowed my son to submit his installation certificate without a problem.
However in Flintshire just our side of the boarder is was very different, in spite of the meters being in full view they still wanted us to get some one else in to test, seemed they had a list of people they trusted, my son said well if we have to get some one in to do testing he would need to hold a better degree to my dad in electrical engineering of course. And at that they backed down and allowed me to test, but not my son, which was a shame as he had insurance and I did not.
So before parting with any money make sure the LABC will accept your paperwork, and if they want calibration certificates be aware these can cost a lot of money. Clearly some one doing it every day needs the meters, but it may be cheaper to use some one from the council list of tester rather than do that bit yourself.
